Thank you, danke, gracias - however you say it, say it often.
As a marketer I am often asked to assist clients find new customers. Which of course, I just love, but sometimes a client doesn't need any new customers, they just need to look after the ones they have.
Saying thank you to customers is a very powerful gesture. You don't have to spend a fortune, you don't have even have to engage a marketing consultant to assist you. Take it from Nike and 'just do it'.
I could ramble on and on with examples of how well it works, how we should be working it into our schedules, how little it costs to keep a customer compared to getting a new one, but I won't. I'm off to write some thank you cards.